Fluid power converters have been known for many years and separate classifications in the Patent Office have been developed to accommodate the various types of fluid power converters.
However, for applications in the low and medium pressure range (up to 1500 psi), gear pumps and motors are the most common types. All but a few are designed to operate above 500 rpm. To drive a 24" diameter wheel at 5 mph requires a rotation of 70 rpm. A hub motor without gear box intended for low angular rates is an expensive device. Unless high precision and special care are used in manufacture, even internal gear motors of the "gerotor" type will not run without "gallop" below minimal effective speeds of 150 rpm.
There is available on the market a small hydraulic motor designed to run at low speeds (down to 0 rpm) with high torque. It is similar to other torque motors well known in the art, and its construction is complex and costly. The elements are machined with extremely close tolerances, and are precision matched. Nevertheless, some power is lost in "slippage."
"Slippage" is the undesirable passage of fluid between input and output areas in the clearance spaces between the housing and the shaft-coupled members. Clearance and fluid viscosity are the most important factors to control "slippage." Closure of clearance spaces by seals created by rotators in rolling contact is accomplished in this invention. The benefit of such seals is compounded by reduction of tolerance demands when they are made resilient (see U.S. Pat. No. 3,653,791). Relatively coarse tolerances facilitate mass production of motor elements when their configuration favors inexpensive fabrication, as by casting, extrusion, or injection molding.
In the following description, the term "rolling seal" is sometimes used. This does not mean that the seal rolls but it is to be understood as meaning a fluid-seal created at the line of contact between a pair of adjacent rotators of similar configuration. This structure is contrasted to a seal created between a stationary surface and a second moving surface wiping along the first surface, as shown in prior art.
It is created between mated pairs of rotators of similar surface characteristics (see FIG. 2 at 39 and 40).
It is distinguished from a displacement path which is created between unmatched pairs of rotators of dissimilar surface characteristics (see FIG. 2 at 48 and 49).